Knowledge checks


Reliable data transfer protocol mechanisms.



Consider the purposes/goals/use of different reliable data transfer protocol mechanisms. For the given purpose/goal/use match it to the RDT mechanism that is used to implement the given purpose/goal/use.



Question List:
Answer List:
  1. NAK
  2. ACK
  3. Checksum
  4. Sequence numbers
  5. Retransmission

 

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1/14

The rdt 2.0 protocol.



Consider the rdt 2.0 sender and receiver shown below, with FSM transitions at the sender labeled S1, S2, and S3; and receiver transitions labeled R1 and R2.

Which of the following sequences of transitions could possibly occur as a result of an initial rdt_send() call at the sender, and possible later message corruption and subsequent error recovery.




 

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2/14

The rdt 2.1 protocol (a).



Consider the rdt2.1 sender and receiver FSMs shown below, with labeled transitions S1 through S6 at the sender, and transitions R1 through R6 at the receiver. The sender and receiver start in the “Wait for call 0 from above” and “Wait for 0 from below” states, respectively.


Suppose that no channel errors occur.  A sequence of interleaved sender and receiver transitions is given below. Transitions S1 and S4 are already provided.  Choose the sender or receiver transition for the unlabeled transitions x1, x2, x3, and x4 below to indicate the time-ordered sequence of transitions (interleaved sender and receiver transitions) that will result in two messages being delivered at the receiver, with the sender and receiver returning to their initial states (again, given that no channel errors occur). 

S1, x1, x2, S4, x3, x4




Question List:
Answer List:
  1. R1
  2. S6
  3. R4
  4. S3

 

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3/14

The rdt 2.1 protocol (b).



 Consider the rdt2.1 sender and receiver FSMs shown below, with labeled transitions S1 through S6 at the sender, and transitions R1 through R6 at the receiver. The sender and receiver start in the “Wait for call 0 from above” and “Wait for 0 from below” states, respectively.


Suppose that the initial message transmission by the sender is corrupted, but that no other message transmissions are corrupted.  Match the unlabeled transitions x1, x2, x3, x4, x5 in the time-ordered sequence of transitions below (interleaved sender and receiver transitions) that will occur following the initial S1 transition (which is corrupted), that will result in two messages being delivered at the receiver, with the sender and receiver returning to their initial states (again, given that the initial message transmission by the sender is corrupted). Note that transitions S1, S4, and S6 are already provided below.

S1 (message corrupted), x1, x2, x3, x4, S4, x5, S6.



Question List:
Answer List:
  1. S2
  2. S3
  3. R4
  4. R6
  5. R1

 

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4/14

The rdt 2.1 protocol (c).



 Consider the rdt2.1 sender and receiver FSMs shown below, with labeled transitions S1 through S6 at the sender, and transitions R1 through R6 at the receiver. The sender and receiver start in the “Wait for call 0 from above” and “Wait for 0 from below” states, respectively.


Suppose that the first packet from the sender is correctly received at the receiver but that ACK message sent from receiver-to-sender is corrupted; all other messages (before or after that ACK) are transmitted error-free.  Match the unlabeled transitions x1, x2, x3, x4, x5 in the time-ordered sequence of transitions below (interleaved sender and receiver transitions) that will occur following the initial S1 transition, which is followed by a corrupted ACK transmission, that will result in a message being delivered at the receiver, with the sender and receiver returning to their initial states. Note that some transitions are already provided below.

S1, x1 (ACK corrupted), x2, x3, x4, S4, x5, S6.




Question List:
Answer List:
  1. R1
  2. R4
  3. R3
  4. S2
  5. S3

 

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5/14

Cumulative ACK.



What is meant by a cumulative acknowledgment, ACK(n)?




 

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6/14

Stop-and-wait: channel utilization.



Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms.  What is the maximum channel utilization of a stop-and-wait protocol for this channel?




 

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7/14

Channel utilization with pipelining.



Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms.  What is the channel utilization of a pipelined protocol with an arbitrarily high level of pipelining for this channel?




 

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8/14

Channel utilization with pipelining (more).



Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms.  How many packets can the sender transmit before it starts receiving acknowledgments back?




 

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9/14

Pipelining.



Which of the following statements about pipelining are true?  One or more statements may be true.




 

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10/14

Packet buffering in Go-Back-N.



What are some reasons for discarding received-but- out-of-sequence packets at the receiver in GBN? Indicate one or more of the following statements that are correct.




 

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11/14

Packet buffering in Go-Back-N (more).



What are some reasons for not discarding received-but- out-of-sequence packets at the receiver in GBN? Indicate one or more of the following statements that are correct.




 

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12/14

Receiver operation in Selective Repeat.



In the SR receiver window (see diagram below, taken from PPT slides and video), why haven’t the red packets been delivered yet? Check the one or more reasons below that apply.





 

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13/14

Receiver operation in Selective Repeat (more).



In SR, why does the receiver have to acknowledge packets with sequence numbers that are less than (and to the left of) those in its window, which starts at rcv_base.




 

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14/14

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